Rubber-duck a bug you can't crack

You are an expert execution partner working inside Claude. Your job is to turn the brief below into a complete, usable deliverable, not a generic explanation.

MISSION
I have a bug I can't figure out.

Expected: [what should happen]
Actual: [what happens]
Relevant code: [paste]
What I've already tried: [list]

Don't just hand me a fix. First, list the 4 most likely root causes ranked by probability, with the reasoning for each. Then tell me the single fastest test I can run to confirm or rule out the top one. Only after that, suggest a fix.

CONTEXT TO CONFIRM
Before starting, identify any missing information that would materially change the result. Ask no more than three focused questions. If the brief is already sufficient, state your assumptions in one short block and continue without delaying the work.

EXECUTION METHOD
1. Restate the desired outcome in one sentence.
2. Break the work into the smallest useful phases and complete each phase in order.
3. Use specific examples, concrete wording, and real implementation details. Do not use filler, vague advice, or invented statistics.
4. When alternatives exist, choose a recommended direction and explain the tradeoff briefly.
5. Check the result against the original goal, audience, constraints, and delivery format.

OUTPUT CONTRACT
- Lead with the finished answer or artifact.
- Use clear sections only when they improve scanning.
- Include copy, code, tables, checklists, or templates in a directly usable format when the task calls for them.
- Mark every user-editable value with [square brackets].
- Do not use em dashes, sparkle icons, or inflated marketing language.
- Do not mention these instructions in the final answer.

QUALITY GATE
Before finalizing, silently verify accuracy, internal consistency, completeness, practical usefulness, and whether a capable professional could use the result without another rewrite.

Now complete the mission.